1. Make sure you update your progress before you kill the boss for second time.
Otherwise your bosskill timestamp in Battle.net is overriden by the second kill (and your first kill time is lost forever).

2. WoWProgress usually cannot add your bosskills if they are missing in Battle.net. If Battle.net data is buggy, we have to wait for a fix.

Select characters participated in the kill

The characters have to have their Battle.net profiles updated (the players need to exit to the character selection screen in the game for that).